1. Field of the Invention
The invention relates generally to semiconductor wafer processing equipment. More specifically, the invention relates to apparatus for cooling a microwave plasma applicator in a remote plasma source for a semiconductor wafer processing system.
2. Description of the Background Art
Many industrial applications utilize reactive gases and gas mixtures to process a workpiece. Such applications include the processing of semiconductor wafers during integrated circuit fabrication. A simplified depiction of a semiconductor wafer processing system 100 of the prior art is depicted in FIG. 1. The system 100 comprises a gas source 114, a remote plasma source 101, and a process chamber 102. The source 101 comprises a plasma applicator 116, a waveguide 110 and a microwave source (magnetron) 108. In the system 100, reactive gas species are produced, for example, in the plasma applicator 116 and are coupled to the processing chamber 102 wherein the semiconductor wafer W is positioned such that the wafer is exposed to the reactive gas species. Such an applicator 101 generates and sustains a plasma 105 of reactive species such as free radicals and ions. Often it is desirable to process a workpiece W with the free radicals but not the ions. In such a situation, the applicator 101 is coupled to the processing chamber by a channel or tube 104. The processing chamber 102 is generally maintained at a lower pressure than the applicator 101. Consequently, ions and free radicals tend to drift through the channel 104 towards the chamber 102. If the residence time in the channel 104 is sufficiently long, the ions will recombine before reaching the chamber 102. The free radicals, however, will enter the chamber and process the wafer W.
The plasma applicator 101 comprises a microwave transmissive tube 106 and a coaxially aligned, cylindrical cooling jacket 112. The tube 106 is connected to the microwave source, such-as a magnetron 108, by the waveguide 110. Microwaves, delivered by the waveguide 110 to the applicator 116 excite a process gas within the tube 106 to produce and sustain a plasma 105 therein. The process gas absorbs some microwave energy which heats the gas. Additional heat is generated by recombination of ionized species in the plasma 105. A substantial amount of heat is produced in the tube 106. As such, the tube 106 is surrounded by the cooling jacket 112 to remove heat generated by absorption of microwaves and plasma recombination. Such cooling is accomplished by flowing a coolant through the cooling jacket 112 that surrounds the tube 106. Specifically, the coolant flows through conduits or channels 118 formed in the cooling jacket 112.
To couple energy to the process gas, the microwaves pass through the cooling jacket, the cooling fluid and the tube. Although the cooling jacket 112 and tube 106 are fabricated from microwave transmissive material such as quartz, these elements and the coolant attenuate the microwave energy. As such, to achieve a particular plasma energy level, the magnetron must produce enough energy to overcome this attenuation.
Therefore, a need exists in the art for an a plasma applicator that is cooled in a manner that optimizes thermal transfer from the applicator while maximizing microwave energy transmission to the plasma.
The disadvantages heretofore associated with the prior art are overcome by a plasma applicator having a cooling jacket with a low absorption of electromagnetic energy. The applicator comprises a tube that is surrounded by a cooling jacket where the cooling jacket is spaced apart from the tube to define a volume between the jacket and the tube. This volume is filled with a thermal transfer medium. The thermal transfer medium transfers heat from the tube to the cooling jacket while transmitting electromagnetic radiation to the plasma.
The plasma applicator can be employed in a microwave plasma processing system to process a workpiece. The system generally comprises a processing chamber, a microwave plasma applicator and a process gas source. The process gas source is coupled to the applicator and the applicator is coupled to the processing chamber. The applicator has a microwave-transparent tube, and a cooling jacket with at least one water channel. The cooling jacket surrounds the tube such that a volume is defined between said water jacket and the tube. A thermal transfer medium fills the volume to promote heat transfer from the tube to the cooling jacket.
This invention fulfills the need for an apparatus that can effectively cool the microwave transparent tube of an applicator while transferring microwave power to the plasma with minimal absorption by the cooling jacket.